Magnetic field dependent cycloidal rotation in pristine and Ge-doped CoCr2O4

We report a soft x-ray resonant magnetic scattering study of the spin configuration in multiferroic thin films of Co0.975Ge0.025Cr2O4 (Ge-CCO) and CoCr2O4 (CCO) under low and high magnetic fields from 0.2 to 6.5 T. A characterization of Ge-CCO at a low magnetic field was performed, and the results were compared with those of pure CCO. The ferrimagnetic phase transition temperature T-C approximate to 95 K and the multiferroic transition temperature T-S approximate to 27 K in Ge-CCO are comparable with those observed in CCO. In Ge-CCO, the ordering wave vector (qq0) observed below T-S is slightly larger compared with that of CCO, and unlike CCO, the diffraction intensity consists of two contributions that show a dissimilar x-ray polarization dependence. In Ge-CCO, the coercive field observed at low temperatures was larger than the one reported for CCO. In both compounds, an unexpected reversal of the spiral helicity, and therefore the electric polarization, was observed on simply magnetic field cooling. In addition, we find a change in the helicity as a function of momentum transfer in the magnetic diffraction peak of Ge-CCO, indicative of the presence of multiple magnetic spirals.